For network operators and end users, Ethernet is becoming ubiquitous. From a network operation view, Ethernet is typically provided as a Layer 2 protocol transported over Layer 0/1 (e.g., Synchronous Optical Network (SONET), Synchronous Digital Hierarchy (SDH), Optical Transport Network (OTN), Generic Framing Protocol (GFP), Wavelength Division Multiplexing (WDM), etc.). As such, a rate mismatch can occur between the Ethernet and the underlying transport mechanism. In accordance with IEEE 802.3x and variants thereof, an overwhelmed network element (NE) can send a PAUSE frame, which halts the transmission by the sender for a specified period of time. Media Access Control (MAC) frames are used to carry the PAUSE commands between NEs. A PAUSE frame includes a period of pause time being requested, in the form of two byte unsigned integer (0 through 65535). This number is the requested duration of the PAUSE. The pause time is measured in units of pause “quanta,” where each unit is equal to 512 bit times. The receiver will honor the most recent PAUSE request it receives and hold off transmission for duration of the pause. Conventionally, there are two implementations for flow control, namely XON/XOFF and periodic quanta.
For XON/XOFF, when a network element's ingress buffer fills to a certain water-mark, a PAUSE frame is sent with a very large pause quanta, with the intent to shut of the far end transmission of frames. When the fill level of the buffer drops below a water-mark, a PAUSE frame is sent with a zero quanta value, with the intent to turn the far end transmitter back on. The goal here is to avoid buffer over-flow and under-run. However, XON/XOFF is severely limited by link distance and the resulting delay for the PAUSE command. Also, this limitation gets worse when the server layer rate is much smaller than the packet data rate. For long link distances and relatively small container sizes, it can be shown large amounts of buffer is required. In addition to limiting link distance, XON/XOFF causes an increase in latency variation (jitter). For example, a gigabit Ethernet (GbE) with GFP-Frame (GFP-F) mapping into an Synchronous Transport Signal-3 concatenated (STS-3c) container and a 54 kB exemplary buffer only has a maximum link distance of about 10 km with a latency variation of 2.88 ms. For periodic quanta, a network element is constantly sending PAUSE frames to its link partner at a predetermined constant quanta and frequency. The quanta and frequency values are chosen in such a way as to insure that the buffer never overflows, but is still provides sufficient throughput. Unlike the XON/XOFF, the periodic quanta works around the link distance limitation, and does not have the issue with latency variation. However, periodic quanta is inefficient, especially for small frames when doing GFP-F mapping of Ethernet.